Our objective is to develop ultrasound pulse Doppler equipment capable of imaging blood flow in aorto-coronary artery bypass grafts. Equipment we will use is the infinite gate pulse Doppler we have previously built, in a form adapted to imaging flow in the anterior mediastinum. Following the equipment development phase, the ability of the system to measure absolute flow velocities or volume flows will be assessed in vitro and using an animal model. Comparisons between this new technique and angiography will be made in a series of patients studied during routine follow-up examinations after surgery. We will also assess the utility of the system in following flow changes in the immediate postoperative period and subsequently using the volumetric flow measured in the operating room as a baseline reference. If the work on the grafts is sufficiently promising, we will determine the feasibility of detecting flow in the natural coronary arteries with this system, or a modified system.